1. Field of the Invention
The invention relates to the field of implantable medical devices and in particular to systems and methods of preventing damage to output stage components of an implantable medical device and providing back-up or safe mode operation in case of a shorted output circuit.
2. Description of the Related Art
A variety of implantable medical devices are known to automatically monitor a patient's physiologic condition and to selectively provide therapy when indicated. Implantable cardioverter defibrillators (ICDs) and/or pacemakers are implantable medical devices which are configured to monitor a patient's cardiac activity and selectively provide therapy for a variety of potentially dangerous or life-threatening medical conditions such as cardiac arrhythmias and/or atrial or ventricular fibrillations. Implantable ICDs typically include a high voltage circuit for generating high voltage waveforms for delivery to patient tissue and a microprocessor-based controller which regulates the delivery of the high voltage waveform. The high voltage circuit and the controller circuitry are generally encased within a biocompatible can or housing along with a battery to power the device.
Implantable ICDs and/or pacemakers typically also include one or more implantable patient leads with associated electrodes. The patient leads include insulated conductors connected at one end to a corresponding electrode and at the other to the high voltage circuitry and controller in the can or housing. The patient leads are frequently configured for transvenous catheterization to place the electrodes into contact with the patient's cardiac tissue.
The leads also typically include relatively thin insulated conductors and are made to be at least partially flexible to accommodate passage through curved vein sections, as well as to accommodate internal movement within the veins and more particularly adjacent the beating heart. The thin nature of the conductors, as well as the constant exposure to vibration and physical movement due to the beating heart muscle as well as other patient movement, can result in lead abrasion or in some cases degradation of the insulation within the lead.
Degradations in the insulative material and lead abrasions tend to result in problematic conductive paths between the distal electrodes and the implantable device housing and in some cases may create a complete short circuit along the conductive path. If a short circuit is formed between one of the distal electrodes and the implantable device housing, the current of a defibrillation shock running through the path may be above the tolerance of some of the vital components of the implantable device and may thus result in irreparable damage to these components. These components generally include output stage transistors that generate the defibrillation waveform.
Damage to these transistors can result in inoperability of the device such that it is no longer able to provide therapy. Furthermore, subsequent charging of capacitors in the high voltage circuit can generate leakage currents that might get injected into the patient's tissue and in some instances induce a fibrillation arrhythmia.
In order to prevent induced arrhythmias, some implantable ICDs include a shorted output stage detection algorithm that monitors various components of the high voltage circuit and turns off the high voltage charging upon detecting a damaged component. This feature decreases the risks of induction. The shorted output stage detection algorithm, however, acts upon detection of a damage and does not assist in preventing the damage.
In order to prevent damage to the components, some implantable ICDs include a shorted output stage prevention algorithm that can detect a short circuit and turn off the high voltage function before the components are damaged. Thus, the shorted output stage prevention algorithm can help prevent damage to some vital components. However, because upon detection of a short circuit, the high voltage functions are aborted, the patient may be left without therapy for an unacceptable period of time. This may lead to injury especially in instance where if the patient was in fibrillation when the short circuit was detected.